Chemical-mechanical planarization ("CMP") processes remove material from the surface of substrates, such as field emission displays and semiconductor wafers. CMP processing, for example, is extensively used to form structures and create flat surfaces in the production of ultra-high density integrated circuits on semiconductor wafers. In a typical CMP process, a wafer is pressed against a non-abrasive polishing pad in the presence of an abrasive slurry under controlled chemical, pressure, velocity, and temperature conditions. The abrasive slurry solutions generally have abrasive particles and chemicals to remove material from the substrate surface. Thus, when relative motion is imparted between the substrate and the pad, the slurry solution removes material from the surface of the substrate.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus optical or electromagnetic circuit patterns on the surface of the wafer. For example, as the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-pattern to within a tolerance of approximately 0.1 .mu.m. Focusing photo-patterns to such small tolerances, however, is very difficult when the surface of the substrate is not uniformly planar. Thus, to reduce the potential of fabricating defective devices, CMP processes must create highly uniform, planar surfaces on substrates.
In the competitive semiconductor industry, it is also desirable to maximize the throughput of the finished wafers and to minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate"). Although high polishing rates are generally desirable, it is more difficult to control the planarity of the surface on the substrate at high polishing rates. Accordingly, it is desirable to maximize the polishing rate within controlled limits.
The polishing rate of conventional CMP processes may be increased by increasing the proportion of abrasive particles in the slurry solution. Yet, one problem with increasing the proportion of abrasive particles in colloidal slurry solutions is that the abrasive particles tend to flocculate when they are mixed with some desirable oxidizing and etching chemicals. Although stabilizing chemicals may prevent flocculation of the abrasive particles, the stabilizing chemicals are generally incompatible with the oxidizing and etching chemicals. Thus, it may be necessary to limit the proportion of abrasive particles in the slurry solution.
Additionally, the polishing rate may vary across the face of a substrate because the slurry may not be distributed uniformly across the face of the substrate. In a typical CMP application, the perimeter of the substrate pushes the slurry across the polishing pad, thereby leaving less slurry under the center of the substrate. It will be appreciated that highly abrasive slurries produce even more disparate polishing rates across the substrate than relatively less abrasive slurries. Thus, it may also be necessary to limit the proportion of abrasive particles in the slurry solution to enhance the uniformity of the polishing rate across the substrate.
One desirable solution for limiting the proportion of abrasive particles in the slurry is to suspend the abrasive particles in the pad. Conventional suspended particle pads are made by admixing the abrasive particles into a matrix material made from monomer chains. An ionic adhesion catalyst, such as hexamethyldisalizane, may be used to enhance adhesion between the particles and the monomer chains. After the abrasive particles are mixed into the matrix material, the matrix material is cured to harden the pad and to suspend the abrasive particles throughout the matrix material. In operation, the suspended abrasive particles in the pad abrade the surface of the wafer to mechanically remove material from the wafer.
One problem with conventional suspended particle polishing pads is that the abrasiveness of the planarizing surface of the pad, and thus the polishing rate of a wafer, varies from one area to another across the surface of the pad. Before the matrix material is cured, the abrasive particles commonly agglomerate into high density clusters, causing a non-uniform distribution of abrasive particles in random patterns throughout the pad. Therefore, it would be desirable to develop a suspended particle polishing pad with a uniform distribution of abrasive particles throughout the pad or throughout different planarizing regions on the pad.
Another problem with conventional suspended particle polishing pads is that they may disadvantageously alter the surface of the substrate. As the pad planarizes a substrate, the matrix material adjacent to abrasive particles on the planarizing surface of the polishing pad wears down. As a result, some of the abrasive particles may eventually detach from the pad and travel in the slurry. Abrasive particles attached to the matix material with an ionic adhesion catalyst may also break away from a pad because electrostatic solvents used in slurries may weaken the ionic bonds between the matrix material and the particles. When a large agglomeration of suspended particles breaks away from the pad, it may disadvantageously alter the surface of the substrate. Therefore, it would be desirable to develop a pad that substantially prevents abrasive particles from detaching from the pad.